1. Field of the Invention
The present invention relates generally to novel data storage systems and particularly to direct memory access techniques for storing serial point to point digital encoded data such as those used in the fiber optic multiplexed data acquisition system described in copending application Ser. No. 887,682, filed July 21, 1986 to the present inventors, Earl J. Holdren and Alexander J. Owski entitled "Fiber Optic Multiplexed Data Acquisition System" now U.S. Pat. No. 4,707,823 dated Nov. 17, 1987.
2. Description of the Prior Art:
Multichannel digital data acquisition systems which require transferring data from illustratively a hostile environment to a less hostile environment must have means for capturing and storing transient data. Usually, such systems are devised for capturing and storing translient response information derived during events such as blasting, ballistics, and crash testing. When many channels of data are required, for example, sixty four channels of data, and if the digital data to be transferred is parallel data, having a word size illustratively of 16 bits, then 16.times.64 16 x or 1024 separate parallel lines would be necessary.
To convert the sixty-four channels of parallel data to serial data and then transfer the data to a remote location, reconstruct the data to parallel words and then store the parallel words, presents problems with respect to coordination and circuit noise which could distort the transferred data. Most prior art direct memory access systems, to allow orderly communication between transmitters and receivers in a data link, employ well-defined protocol for management of data. Protocols are used to perform functions such as establishing and terminating messages between two stations, identifying the sender and receiver, acknowledging received information, and initializing stations. Also, in order to permit utilization of various communications systems, accepted standard protocol is normally empolyed. Generally, data communications systems are designed around accepted bit or byte oriented standard protocol such as Synchronous Data Link Control (SDLC) and Binary Synchronous Communication (BSC). SDLC is a bit oriented protocol in which any size data word may be transmitted. It has very flexible provisions for message switching and error control. Communications in the bit oriented protocols usually are in the form of frames of uniform format. The frames comprise a number of fields each having a definite location and precise meaning. BSC is byte oriented, requiring data to be transmitted in multiples of eight bits. Transmission of information in BSC form is usually limited to half-duplex (two-way alternate). This results from the stop-and-wait procedure which characterizes such protocol. Once a communications channel is established and the transmitter sends one block, it stops and waits for an acknowledgement signal before sending another block.
To accommodate the standard protocols, receiver components are designed to respond to the characters presented in the formats. This often entails designing circuits to respond to separate words of routing, destination addressee, and time of origination information or to separate frames of opening flag, address field, control field, information field, frame check sequence field and closing flag information. Devotion of an extensive amount of circuitry to comply with the protocol also slows data communication between the transmitter and receiver and often limits the amount of data that can be processed.
A search was made for means of transferring a series of digital data words representations from one point to a remote transient data recorder that is not burden with responding to extensive protocol requirements. The search resulted in the serial data direct memory access system of the present invention.